Various types of biometric systems are used more and more in order to provide an increased security for accessing an electronic device and at the same time keep the user convenience at an acceptable level. In particular fingerprint sensors have been successfully integrated in such devices, for example, thanks to their small form factor, high performance and user acceptance. Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important.
All capacitive fingerprint sensors provide an indicative measure of the capacitance between several sensing elements and a finger placed on the surface of the fingerprint sensor. Acquisition of a fingerprint image is typically performed using a fingerprint sensor comprising a plurality of sensing elements arranged in a two-dimensional manner, and a block based technique may be applied to the fingerprint sensor for acquiring a fingerprint image, where the blocks of sensing elements are sampled sequentially.
One of the problems associated with fingerprint sensors concerns the presence of fixed patterns, resulting from the manufacturing of the fingerprint sensor or formed during use of the sensor, including for example scratches, etc. Such fixed patterns effectively result in degraded biometric performance.
A special case is when fixed patterns are introduced after the enrollment phase and after the security level of the system has been determined. Due to the need to accommodate ageing of biometric traits, the fingerprint template will likely have to be updated over time. This leaves the system vulnerable to a fixed pattern being introduced into the biometric fingerprint template.
In consequence, a lower than expected security level will result, since the matching process may recognize fixed pattern components in imposter attempts, thereby leading to a higher than expected false accept rate. Even without progressive updating of the fingerprint template, a fixed pattern may enter the template if the (genuine) user chooses to re-enroll. This re-enrollment may for example be triggered as a result of poor biometric performance due to the reduced similarity between the template and the identification samples, as the latter group now contains a fixed pattern in addition to fingerprint data.